Tools > Accounts
Add new mail account - enter [from/reply to] email address and password and clear the 'Configure automatically'
- User name (without the @yahoo.com bit)
- Password (should be from initial dialogue)
- Incoming server: imap.mail.yahoo.com and check the Use SSL (should default to Port 993)
- Outgoing server: smtp.mail.yahoo.com and check Override default port and Use SSL; set port to 465
That should be it, although to keep everyone happy, you might want to confirm your account name/password under outgoing server's 'More options'.
I actually have my email address set to something else (a 'lifetime' email forwarding alias) and Yahoo seems pretty happy with that - although that's probably another benefit of paying to use the service.